The present invention relates to aliphatic, light-stable, sinterable, thermoplastic polyurethane molding compositions with improved blooming behavior, good thermal stability and good technical processability.
Thermoplastic polyurethanes (TPU) are of great technical importance because of their good elastomer properties and thermoplastic processability. An overview of the production, properties and uses of TPU's is given, for example, in Kuntstoff Handbuch [G. Becker, D. Braun], Vol. 7, “Polyurethane”, Munich, Vienna, Carl Hanser Verlag, 1983.
TPU's are in most cases built up from linear polyols (macrodiols), such as polyester, polyether or polycarbonate diols, organic diisocyanates and short-chain, for the most part dihydric alcohols (chain extenders). They may be produced continuously or batch-wise. The best known production processes are the belt process (GB-A 1 057 018) and the extruder process (DE-A 19 64 834).
The synthesis of the thermoplastically processable polyurethane elastomers may be carried out either stepwise (prepolymer metering process) or by the simultaneous reaction of all components in one stage (one-shot metering process).
In the production of aliphatic thermoplastic polyurethanes based on 1,6-hexamethylene diisocyanate (HDI), cyclic oligourethanes are formed. Due to the specific crystallization process of these cyclic oligourethanes, they are less compatible with the polymer matrix and therefore lead to the formation of a chalky interfering surface coating. This phenomenon is described, for example, in DE-A 102 06 839. It has been shown that test storage at room temperature (100 days) or 28 days at 60° C. in an environment saturated with water vapor cannot provide a sufficiently accurate prediction of the long-term behavior. For this reason, accelerated water storage tests are also carried out in order to be able to estimate better the blooming behavior over a longer time period.
In principle, the consistency and thus also the blooming behavior of the cyclo-oligourethanes can be altered either by changing the diisocyanate and/or adapting the chain extender. Because of the good thermal stability, the mechanical properties and the good stabilization behavior, HDI is however particularly suitable as diisocyanate component for the production of TPU parts in automobile interiors (e.g., as instrument panel surfaces or other surfaces), so that a change of the diisocyanate is virtually out of the question. On the other hand, the chain extenders have been adapted with regard to the aforementioned requirements.
The use of 1,6-hexanediol, 1,5-pentanediol, 1,4-cyclohexanediol, 1,4-bis(hydroxymethyl)cyclohexane and of bis(hydroxyethyl)hydroquinone to improve the blooming behavior is described in DE-A 100 50 495.
The use of ethylene glycol, 1,4-bis(hydroxymethyl)benzene, 1,4-bis(hydroxyethyl)benzene and 1,4-bis(2-hydroxyethoxy)benzene to avoid the formation of a surface deposit is described in DE-A 100 37622.
The aforementioned TPU's, however, exhibit an undesirable blooming behavior after a fairly long period, which is undesirable especially if the TPU's are employed in automobile interiors.